It looked to me like the oval shields from Duras-Europos were made just from butted planks without a reenforcement to hold the planks together (such as how viking shields are supposed to have three strips of iron across the back). How would the shield be held together without these reenforcements? Does anyone have any experience making a shield like this, or have idea as to how I could do this?

The Dura Europos shields, like all late Roman and early medieval shields are made from a single layer of thin planks with butted edged which are glued together, presumably with a cassein glue (which does an excellent job, creating a join that can be stronger than the planks). The planks are thin and often tapered towards the edges.

The boards are then covered front and back with either thin leather or (more likely) thin untanned hide. Not only is hide tougher and harder to penetrate than the same thickness of leather but also, when oiled, it becomes transparent which would allow any decoration on the boards to show through. It is the casing of hide which provides the durability to the planked core, which itself provides flexibility. It's this combination which makes the shield effective. It's not a barn door, solid and unyielding, it's a flexible thing which absorbs energy through it's ability to deform under impact.

Some 3rd-7th century shields had very thin metal rims (see some of the Danish bog shields or the Sutton Hoo shield) but whether or not a separate organic rim was applied to others is debatable. Pictorial evidence indicates that the rear facing may have been folded over and stitched over the front around the edge, creating the appearance of a rim from the front. The original excavators of the Dura shields stated that they saw a separate hide rim on one of the shields but there was no evidence of this when James studied them for his report.

As an aside, there's no proof that "Viking" shields had three iron strips across the back. The Gulathing law just states that a shield "must have three small plates of iron laid across it", giving no indication as to size or whether they are on the front or the back.

Thanks Matt (Paul here by the way). As you know, I've been planning to create an early medieval planked shield. Most of the references I've seen have been frustratingly Indistinct as to whether the backing and front are rawhide or leather ( as you indicated).

Would there be any practical benefit in a front of rawhide, with a leather backing to provide a small absorption of the kinetic energy hitting a shield ? Or would a rawhide front and back be more effective? ( by the way, that would be quite an interesting bit of experimental archaeology). I'm not convinced that a leather front and back would be effective ( but would be happy to be proved wrong).